The present invention generally relates to spin-transfer torque magnetic random access memory (STT MRAM) devices, and more specifically to perpendicular magnetic anisotropy (PMA) materials in STT MRAM devices.
A STT MRAM device is a type of solid state, non-volatile memory device that uses tunneling magnetoresistance (TMR or MR) to store information. MRAM includes an electrically connected array of magnetoresistive memory elements, referred to as magnetic tunnel junctions (MTJs). Each MTJ includes a free layer and fixed/reference layer that each include a magnetic material. The free and fixed/reference layers are separated by a non-magnetic insulating tunnel barrier. The free layer and the reference layer are magnetically decoupled by the tunnel barrier. The free layer has a variable magnetization direction, and the reference layer has an invariable magnetization direction.
The MTJ stores information by switching the magnetization state of the free layer. When the free layer's magnetization direction is parallel to the reference layer's magnetization direction, the MTJ is in a low resistance state. Conversely, when the free layer's magnetization direction is antiparallel to the reference layer's magnetization direction, the MTJ is in a high resistance state. The difference in resistance of the MTJ indicates a logical ‘1’ or ‘0’, thereby storing a bit of information. The TMR of an MTJ determines the difference in resistance between the high and low resistance states. A relatively high difference between the high and low resistance states facilitates read operations in the MRAM.
The magnetization direction of the free layer may be changed by a spin-transfer torque (STT) switched write method, in which a write current is applied in a direction perpendicular to the film plane of the magnetic films forming the MTJ. The write current transfers spin angular momentum to the free layer which creates a torque to change (or reverse) the free layer's magnetization direction. During STT magnetization reversal, the write current for magnetization reversal is determined by the current density. As the surface area of the the MTJ becomes smaller, the write current for reversing the free layer's magnetization becomes smaller. Therefore, if writing is performed with fixed current density, the necessary write current becomes smaller as the MTJ size becomes smaller.
Compared to MTJs with in-plane magnetic anisotropy, layers with perpendicular magnetic anisotropy (PMA) can lower the necessary write current density. Thus, PMA materials lower the total write current used.